The present invention relates generally to modem relay network systems such as Modem over Internet Protocol (MoIP) systems. More particularly, it concerns method and apparatus for providing a fallback mode of V.14 or other synchronous modem operation in case one or more of two modem terminal devices is a non-V.42-type modem or is a V.42-type modem configured not to follow the V.42 link layer protocol.
V.42 Modem Relay gateways are used for transport of modem data over Voice over Internet Protocol (VoIP) networks. Typically, this involves three legs: Originating Telephony Leg, Terminating Telephony Leg, and an Internet Protocol (IP) Leg.
Originating Telephony Leg consists of the telephony leg between the originating modem and its associated gateway. Terminating Telephony Leg consists of the telephony leg between the answerer modem and its associated gateway. The IP Leg refers to the connection over the IP network joining the two telephony legs.
Frequently, both of the two telephony legs run the V.42 link layer protocol. This is frequently the case with dial-up Internet connections. In this case, a “V.42 modem relay gateway” would terminate the physical layer as well as V.42 error correction layer over the telephony legs. It may additionally terminate the compression layer.
Information that a gateway receives on its telephony leg from the client modem on its telephony leg can then be sent over a reliable IP transport protocol to the remote gateway, which would then forward this information to its associated client modem.
The procedure for two modems (e.g. a client modem and its associated gateway modem) to negotiate V.42 link layer is done shortly after physical layer bring-up, using ADP/ODP sequences exchanged between the two modems. However, either of the two modems may be configured to not use any link layer protocol. For example, one side may not generate the standard ADP or ODP patterns, whether by manual intervention or otherwise. In this case, the V.42 specification has defined a fallback mechanism to a no-error-correction mode of operation, based on the V.14 specification.
The V.14 mode of operation involves transfer of data using a “Start/Stop” delimited character format. A typical application is gaming, which may run the PPP protocol straight over non-error-corrected V.14 modem links. The V.14 mode is typically used in gaming due to its reduced latency, since information is transmitted in characters and hence there is no packetization delay. Data errors are acceptable in such an application whereas latency, i.e. transmission or turn-around delays, is not.
The case where modem relay techniques support the non-V.42 mode of operation (which is mandatory as per the V.42 specification) is significantly different from the case where both telephony legs use V.42 link layer protocol. The present invention provides a modem relay approach to support a non-V.42 mode of MoIP operation.